ECL in a recording and reproduction device in which coefficient groups including coefficient data corresponding to low frequency components of the first axis in at least two overlapped coding blocks are not recorded by the same recording head

ABSTRACT

In a recording and reproducing apparatus with image coding for compression coding of digital video signals and for recording and reproducing the compressed code, digital video signals are divided on a screen into coding blocks which are overlapped so as to share boundary pixels. DCT coefficient data in each coding block are divided into a plurality of groups, and coefficient data in two coding blocks overlapping on the screen are recorded to separate regions on a recording medium. When an error has occurred during a reproducing process, the state of the error occurrence is analyzed and an error correction responsive to the state of error occurrence is performed on coefficient data that has been lost due to the error. Thus, a stable, successful reproduced image can be obtained even if an error occurs during the recording and reproducing process.

TECHNICAL FIELD

The present invention relates to a recording and reproducing apparatusfor recording digital video signals subjected to compression coding forimage coding.

BACKGROUND ART

In recent years, a variety of digital instruments have been developed asdigital signal processing techniques are improved. Small-sized digitalvideo recorders for recording digital video signals in the form ofcompressed codes have been realized lately. Among various methods thathave been proposed for the compression coding of digital video signals,two-dimensional discrete cosine transform (DCT) coding, which is one ofthe methods for frequency conversion of digital video signals in theunit of a block, has become the mainstream as used in JPEG and MPEG. Inthis method, digital video signals are divided into a plurality ofcoding blocks and discrete cosine transform is performed for each codingblock, and further the resulting coefficient data are subjected tovariable-length coding for each block, to compress data.

However, in such a recording and reproducing apparatus using compressioncoding, if an error has occurred to data during the recording orreproducing process, variable-length codes in a certain range subsequentto the error occurrence point could not be decoded. Also, even if thevariable-length decoding is refreshed by detecting a break in thevariable-length code by means of refresh codes or the like, thevariable-length codes themselves over a range from the occurrence of anerror to the refreshing could not be decoded. In such a case, the imagewould drop out in the unit of the block on the reproduction screen, andthe dropping out would succeed over several blocks.

In order to solve this problem in the compression coding, in therecording and reproducing apparatus with image coding described inJapanese Patent Laid-Open Publication 7-298194/1995 by the presentinventors, errors are corrected by using coefficient data of neighboringcoding blocks having high spatial correlation. In this error correction,for recording, coefficient data within one coding block are mixed withcoefficient data of a boundary portion within a neighboring coding blockand subjected to shuffling. In the shuffling, it has also been proposedto divide neighboring coding blocks so that their boundary portion isshared therebetween. Then, the discrete cosine transform is performedwith respect to coding blocks constituting the same region on thescreen, the resulting coefficient data are classified into groups, andthe groups are recorded in separate regions on the recording medium. Inthis apparatus, even if an error occurs during recording or reproducingprocess, there will never occur any collective disappearance ofassociated coefficient data. Further, even if coefficient data hasdisappeared upon occurrence of an error, correct data can be restoredmore accurately by using the correlation of coefficient data. Therefore,the stability of recording and reproducing operations can be greatlyenhanced without causing the image to disappear in blocks.

However, even in this recording and reproducing apparatus with imagecoding, if a plurality of errors of coefficient data have occurredwithin the same coding block, correct restoration by error correctioncannot be done. It is desirable that an error can be corrected so thatimage is not dropped out in such a situation as mentioned above.

SUMMARY OF THE INVENTION

An object of the present invnetion is to provide a recording areproducing apparatus with image coding in which an image will never bedropped out in blocks even if a plurality of errors occur when digitalvideo signals are recorded or reproduced with compression coding.

In a recording and reproducing apparatus with image coding according tothe present invention, for recording process, overlap blocking meansdivides input digital video signals into data in two-dimensional codingblocks each having first and second axes and comprises a plurality ofpixel data, in such a way that neighboring coding blocks are overlappedwith each other so as to share at least one boundary pixel in afirst-axis (for example, horizontal) direction. Orthogonal transformmeans performs two-dimensional frequency conversion (for example,discrete cosine transform) for pixel data of coding blocks obtained bythe overlap blocking means to generate coefficient data of individualfrequencies. Shuffling means divides the coefficient data obtained bythe orthogonal transform means into a plurality of coefficient groups sothat the coefficient groups do not have the same frequency component ofthe second (for example, vertical) axis at least in a frequency regionin the coding blocks corresponding to low frequency components of thefirst (for example, horizontal) axis. Recording means locates recordingdata so that the plurality of coefficient groups in the same codingblock obtained by the shuffling means are recorded so as to be dispersedinto separate regions on a recording medium, and it adds errorcorrection codes for the coefficient data.

For a reproducing process, reproduction decoding means reproducesrecording data from the recording medium and decodes the coefficientdata of the individual coefficient groups, while it detects the presenceor absence of an error of coefficient data. Deshuffling means integratescoefficient data outputted from the reproduction decoding means intocoding blocks for recording. When an error has been detected by thereproduction decoding means, error-correction control means outputs anerror-correction control signal based on the number of errors within acoding block and the number of the order of the two-dimensionalfrequencies of coefficient data to which errors have occurred, and errorcorrecting means corrects the errors of the coefficient data by an errorcorrecting method selected by an error-correction control signal of theerror correction control means with respect to the coding block in whicherrors have been detected. block in which errors have been detected.

Inverse orthogonal transform means decodes pixel data by frequencyinverse conversion of the coefficient data that have been errorcorrected by the error correcting means, and relocating means generatesa digital video signal by relocating pixel data to the coding blocks forrecording. When an error has occurred, such a possibility can beprevented that all the coefficient data within a block may be droppedout so as to lack in blocks during reproducing process. Further, thestability of reproduced images can be enhanced by performing errorcorrection with coefficient data of neighboring blocks having highspatial correlation.

Preferably, the recording means locates recording data, so thatcoefficient groups belonging to the same coding block are not recordedto the same recording track on the recording medium. Upon occurrence ofan error, such a possibility can be prevented that all the coefficientdata within the block may be dropped out so as to lack in the unit ofblock during reproducing process.

Preferably, the recording and reproducing apparatus with image codingfurther comprises a rotary cylinder for helically scanning the recordingmedium, and signal recording means having a plurality of recording headsattached to the rotary cylinder. The recording means outputs recordingdata to the signal recording means so that coefficient groups includingcoefficient data corresponding to low frequency components of the first(for example, horizontal) axis within at least two overlapped codingblocks in the overlap relationship are not recorded to the samerecording head. In a different way, the recording means outputsrecording data so that coefficient groups including coefficient datacorresponding to the same horizontal frequency component in twooverlapped coding blocks in the overlap relationship are not recorded tothe same recording head. When an error has occurred, such a possibilitycan be prevented that all the coefficient data within the block maydropped out so as to lack in blocks during reproducing process.

Preferably, the error-correction control means (a) outputs an zero errorcorrection signal for replacing an error coefficient with a zero when aplurality of error coefficients are present in coefficient data havingthe same frequency in the second (for example, vertical) axis in twocoding blocks in the overlap relationship and when all the errors arepresent at numbers of order higher than a specified number of order, (b)outputs an overlap correlation error correction signal for instructingerror correction by making use of correlation between coefficient dataof neighboring coding blocks when a plurality of errors are present incoefficient data having the same frequency in the second (for example,vertical) axis in two coding blocks in the overlap relationship and whenonly one error coefficient is present at numbers of order lower than aspecified number of order, and (c) outputs a three-dimensional errorcorrection signal for replacing all the coefficient data in the codingblock with coefficient data of the preceding frame or preceding fieldwhen two or more errors are present at numbers of order lower than aspecified number of order on a coding block basis and when errorcoefficients are present on a side of lower numbers of order than aspecified number of order in coefficient data having the same frequencyin the,second (for example, vertical) axis on the coding block basis,and outputs a zero error correction signal for replacing the errorcoefficients with zeroes for coding blocks when no error coefficientsare present in the coding blocks on a side of lower numbers of orderthan the specified number of order on the coding block basis. In thisway, an appropriate error correction signal is outputted in response toan error occurrence state. Thus, an error correction responsive to theerror occurrence state can be achieved.

Then, (d) when an overlap relationship error correction signal isreceived from the error-correction control means, the error correctingmeans corrects, by using correlation of pixels overlapped in the firstaxis on the screen, error coefficients corresponding to frequencycomponents of the lowest region of the first (for example, horizontal)axis out of a plurality of error coefficients in coefficient data havingthe same frequency in the second (for example, vertical) axis in twocoding blocks in the overlap relationship, and moreover corrects theremaining error coefficients as zeroes. Further, (e) when a zero errorcorrection signal is received from the error-correction control means,the error correcting means corrects to zeroes all the plurality of errorcoefficients among coefficient data having the same frequency in thesecond (for example, vertical) axis in two coding blocks in the overlaprelationship. Furthermore, (f) when a three-dimensional error correctionsignal is received, the error correcting means replaces all thecoefficient data within a coding block with coefficient data of acorresponding coding block preceding by one field or one frame andconstituting the same region on the screen. In this way, because theerror correcting means receives an appropriate error correction signalresponsive to an error occurrence state, error correction is performedby using coefficient date of neighboring blocks having a high spatialcorrelation, and data are corrected appropriately, by making use of theproperties of the image. Thus, the stability of reproduced images can beimproved. More preferably, the error-correction control means changesthe specified number of the order of frequencies in the first (forexample, horizontal) axis in response to the number of order offrequencies in the second (for example, vertical) axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an overall setup of a recording andreproducing apparatus with image coding according to the presentinvention;

FIG. 2 is a diagram on the concept of overlap block division in anembodiment of the invention;

FIG. 3 is a schematic diagram on a recording area of recording heads ona recording medium in the embodiment;

FIG. 4 is a diagram for explaining the relationship between coefficientdata and coefficient groups of coding blocks in the embodiment; and

FIG. 5 is a diagram for explaining recording positions of coefficientgroups to be recorded on the recording medium as well as the recordingheads.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, a recording and reproducing apparatus with image codingaccording to an embodiment of the present invention is explained withreference to the appended drawings. FIG. 1 is a block diagram showingthe overall setup of a recording and reproducing apparatus with imagecoding. Digital video signals are transmitted to the apparatussuccessively in steps of one frame or field. A digital video signal isinputted to an overlap blocking circuit 2 via an input terminal. Theoverlap blocking circuit 2 is a circuit which divides digital videosignals into coding blocks of a N×M (e.g., N=M=4) two-dimensional matrixso that two coding blocks neighboring on the screen share boundarypixels between them. Generally, when input digital video signals aredivided into two-dimensional coding blocks each comprising a pluralityof pixel data, neighboring coding blocks share at least one boundarypixel therebetween horizontally or vertically and overlap each other. Inthis embodiment, as will be described later with reference to FIG. 4,digital video signals are divided horizontally so that four boundarypixels are shared therebetween in the horizontal direction. A discretecosine transform (DCT) circuit 3 is orthogonal transform means forperforming discrete cosine transform on pixel data of N×M coding blocksoutputted from the overlap blocking circuit 2 to generate coefficientdata in a two-dimensional frequency region.

The coefficient data obtained by the DCT circuit 3 are divided into aform suitable for error correction, and recorded to a recording medium8. A shuffling memory 4 inputs coefficient data of the discrete cosinetransform circuit 3 and temporarily stores them in the form of aplurality of divided coefficient groups having no identical verticalfrequency components at least in a frequency region of horizontal lowfrequency components in the coding block. A variable-length codingcircuit 5 encodes coefficient data in the individual coefficient groupsoutputted from the shuffling memory 4 invariable lengths. A recordingcircuit 6 inputs codes of the variable-length coding circuit 5 andchanges the arrangement of data so that at least coefficient groups inthe same coding block can be dispersed and recorded into separate tracksand regions on the recording medium 8 shown in FIG. 5. Further, it addserror correction codes and converts the data into recording codes, thusoutputting them to recording heads 7 a-d. That is, a plurality ofcoefficient groups in the same coding block are recorded so as to bedispersed into different tracks and regions, respectively.

The recording medium 8 is a magnetic tape for recording and reproducingdigital data. For recording and reproduction, a rotary cylinder 10,recording heads and reproducing heads attached to the rotary cylinder 10are used. The rotary cylinder 10 is intended to helically scan themagnetic tape, and the recording heads 7 a, 7 b, 7 c, 7 d and thereproducing heads 9 a, 9 b, 9 c, 9 d are mounted to the rotary cylinder10. The recording heads 7 a, 7 b scan adjacent tracks, respectively. Therecording heads 7 c, 7 d mounted at positions 180° opposite to therecording heads 7 a, 7 b scan adjacent tracks in succession. Thereproducing heads 9 a, 9 b and reproducing heads 9 c, 9 d are providedto scan the same tracks relatively as the recording heads 7 a-d.

A reproduction circuit 11 demodulates signals read from the reproducingheads 9 a, 9 b, 9 c, 9 d to transform recording codes to original codesand, upon detection of a code error, adds an error-correction errorflag. A variable-length decoding circuit 12 converts a variable-lengthcode demodulated by the reproduction circuit 11 into coefficient dataand, upon occurrence of an error in the coefficient data, adds adecoding error flag. In this case, the reproduction circuit 11 and thevariable-length decoding circuit 12 have accomplished the functions ofreproducing recorded data in the recording medium 8 and decoding thecoefficient data of coefficient groups as well as a further function asreproduction-process decoding means for detecting the presence orabsence of any error in coefficient data. A deshuffliing memory 13integrates the coefficient data decoded by the variable-length decodingcircuit 12 into coding blocks during the recording process, andtemporarily stores the data.

An error-correction control circuit 14 outputs an error-correctioncontrol signal to an error correcting circuit 15 in response to an erroroccurrence state in the coding block in which an error has beendetected. As will be described later, when an error has been detected byreproduction-process decoding means, the error-correction controlcircuit 14 outputs an error-correction control signal based on thenumber of errors that have occurred in the coding block and on thenumber of the order of two-dimensional frequency components of thecoefficient data to which an error has occurred. The error correctingcircuit 15, as will be detailed later, corrects an error of coefficientdata by an error correcting method selected by an error-correctioncontrol signal from the error-correction control circuit 14 on thecoding block in which the error has been detected. A field memory 16stores, in the unit of a field, coefficient data obtained in the unit ofa block and corrected by the error correcting circuit 15, and feeds backthe coefficient data preceding by one frame to the error correctingcircuit 15 when required.

An inverse discrete cosine transform circuit 17 performs a transformprocess inverse to the transform performed by the discrete cosinetransform circuit 3, to output pixel data including brightness data andhue data in each block. An overlap block decomposing circuit 18 returnspixel data divided in the unit of a coding block into digital videosignals of the form in which the data have been inputted on recording,to output the video signals from an output terminal 19. The inversediscrete cosine transform circuit 17 and the overlap block decomposingcircuit 18 function as inverse orthogonal transform means for decodingpixel data by frequency-inverse-transform of reproduced coefficientdata, and another function as a means for relocating these pixel data tothe coding blocks at the time of recording.

The recording and reproducing apparatus with image coding constructed asexplained above is described in its operation with reference to FIGS. 2to 5. For ease of understanding, an example is explained wherein acoding block comprises 4×4 pixels and input digital video signals aresubjected to compression coding in the unit of a field, and data arerecorded to six tracks by using two-channel recording heads provided180° opposite to each other on the rotary cylinder.

When digital video signals are supplied to the input terminal 1 in FIG.1, the overlap blocking circuit 2 delimits the digital video signal, asshown in FIG. 2, so that two 4×4 coding blocks adjacent to each other onthe screen share the four pixels present at their edges, thus dividingthe signals into overlapped 4×4 coding blocks. In FIG. 2, hatchingrepresents the shared pixels. Next, the discrete cosine transformcircuit 3 performs frequency transform on the pixel data for each codingblock to generate two-dimensional coefficient data. Then, thesecoefficient data are stored in the shuffling memory 4.

The variable-length coding circuit 5 encodes coefficient data outputtedfrom the shuffling memory 4 in variable lengths, according topredetermined shuffling rules. The recording circuit 6 adds errorcorrection codes to coefficient data which have been coded in variablelengths, subjects them to recording modulation, and then supplies themto the two-channel recording heads 7 a-d through a recording amplifier.The recording heads 7 a, 7 b, 7 c, 7 d record input coefficient dataonto the recording medium 8. In this process, digital video signals ofone field are recorded to six tracks. Data are recorded to the tracksT1, T2, T3, T4 on the recording medium 8 alternately by the recordingheads 7 a, 7 b and 7 c, 7 d as shown in FIG. 3. The recording heads 7a-d scan the tracks T1 to T6, and record the input one field digitalvideo signals onto the six tracks on the recording medium 8.

Next, the shuffling rules in the shuffling memory 4 are described withreference to FIG. 4. In FIG. 4, (a) illustrates two coding blocks to beoverlapped, comprising of a block of (a₀₀, a₀₁, a₀₂, a₀₃)-(a₃₀, a₃₁,a₃₂, a₃₃) and another block of (b₀₀, b₀₁, b₀₂, b₀₃)-(b₃₀, b₃₁, b₃₂,b₃₃), respectively. In this case, a row (a₀₃, a₁₃, a₂₃, a₃₃) and anotherrow (b₀₀, b₁₀, b₂₀, b₃₀) are overlapping pixels. By discrete cosinetransform of these pixel data, coefficient data A₀₀-A₃₃, B₀₀-B₃₃ shownin (b) are obtained. In the coefficient data, for example, A₀₀ iscoefficient data whose numbers of order of horizontal frequency andvertical frequency are both 0, representing the DC component of theimage, A₀₃ is coefficient data whose number of order of horizontalfrequency is 3 and whose number of order of vertical frequency is 0, andA₃₀ is coefficient data whose number of order of horizontal frequency is0 and whose number of order of vertical frequency is 3.

The shuffling rules in this case are shown in (c). The coefficient datain a coding block is divided into three groups in total. A group G_(A0)comprises four coefficient data (A₀₀, A₁₀, A₂₀, A₃₀) in which the numberof order of horizontal frequency is in the lowest region 0 and which donot include the same vertical frequency, a group G_(A1) comprises ofcoefficient data (A₀₁ A₁₁, A₂₁, A₃₁) in which the number of order ofhorizontal frequency is 1, and a group G_(A2) comprises eightcoefficient data (A₀₂, A₁₂, A₂₂, A₃₂, A₁₃, A₂₃, A₃₃) in which the numberof order of horizontal frequency is as high as 2 and 3. A reason thatthe high-frequency two rows of data are put together into one group isto prevent a decrease the coding efficiency. The neighboring codingblock is, also divided into groups G_(B1), GB_(B2), similarly. It isnoted that FIG. 4 (c) only shows an example of group division and someother type of division may be used. 7 b (or 7 d) that are different fromeach other, respectively, while the coefficient groups G_(A1) andG_(B1), are recorded by the heads 7 d (or 7 b), 7 c (or 7 a) that aredifferent from each other, respectively.

In this way, the shuffling memory 4 performs shuffling in such a waythat six groups G_(A0), G_(A1), G_(A2), G_(B0), G_(B1), G_(B2) in twooverlapping coding blocks are recorded at least to separate tracks andregions on the recording medium 8. In the example shown in FIG. 5, thedata of the groups G_(A0), G_(A2), G_(B1), G_(A1), G_(B2), G_(B0) arerecorded in succession to the tracks T1-T6 to record one field. That is,a plurality of groups are recorded to tracks and regions separate fromeach other, respectively. Further, as shown in FIG. 5, the shuffling isperformed in such a way that the same recording head does not record anycoefficient groups having the same horizontal frequency component in thesame coding block, and that the same recording head does not record thecoefficient groups G_(A0), G_(B0) and the coefficient groups G_(A1),G_(B1) corresponding to a low frequency region in horizontal frequencycomponents. That is, a plurality of recording heads are provided, andone recording head records a coefficient group showing horizontalfrequency components in the same coding block, while another differentrecording head records a coefficient group different from thiscoefficient group. In the example shown in FIG. 5, the recording heads 7a, 7 b, 7 c, 7 d record signals to one track in turn. The coefficientgroups G_(A0) and G_(B0) are recorded by the heads 7 a (or 7 c), 7 b (or7 d) different from each other, respectively, while the coefficientgroups G_(A1) and G_(B1) are recorded by the heads 7 d (or 7 b), 7 c (or7 a) different from each other, respectively.

Next, reproduction of the coefficient data recorded by theabove-mentioned recording process is described. The reproducing heads 9a, 9 b, 9 c, 9 d reproduce the signals recorded by the recording heads 7a, 7 b, 7 c, 7 d from the recording medium 8 and supply the reproductionoutputs to the reproduction circuit 11. The reproduction circuit 11demodulates this reproduction output and performs error correction. Thenit supplies the coefficient data of variable-length coding and theerror-correction error flag as a result of error-correction to thevariable-length decoding circuit 12. The variable-length decodingcircuit 12 performs variable-length decoding of the coefficient data ofvariable-length coding, and outputs a decoding error flag showingwhether or not variable-length decoding has been accomplished correctly,along with the decoded coefficient data, to the deshuffling memory 13.

This decoding error flag is kept to be outputted over an interval untilthe variable-length decoding is refreshed by a refresh code or the likesubsequent to an error occurrence point shown by the error-correctionerror flag outputted by the reproduction circuit 11. That is, thedecoding error flag is outputted for the coefficient data to be decodedduring the interval. In a different way, a decoding error flag isoutputted when a certain number of decoding operations are not performedcorrectly during the interval in which a specified number of coefficientdata should be decoded.

The error-correction control circuit 14 reads, from the deshufflingmemory 13, coefficient data constituting the same region on the screenin the unit of a block and the decoding error flags corresponding to theindividual coefficient data, and outputs an error-correction controlsignal of an error correction method to the error correcting circuit 15.The error correcting circuit 15 corrects errors according to the errorcorrection control signal for a coding block in which an errorcoefficient is present, by using either the inputted coefficient data intwo coding blocks having the overlap relationship, or coefficient datastored in the field memory 16 for a coding block preceding by one fieldand located in the same region on the screen. Then, the coefficient datasubjected to error correction are outputted to the inverse discretecosine transform circuit 17, and the contents of the memory space on thesame region on the screen accumulated in the field memory 16 areupdated.

The coefficient data subjected to error correction are transformed intopixel data by the inverse discrete cosine transform (inverse DCT)circuit 17, and supplied to the overlap block decomposing circuit 18.The overlap block decomposing circuit 18 restores the pixel data dividedin the unit of a coding block to digital video signals in the form inwhich the data have been inputted via the input terminal 1 on recording,and then outputs the signals to the output terminal 19.

Now, the operations of the error-correction control circuit 14 and theerror correcting circuit 15 are described in more detail. As shown inFIG. 4, two coding blocks in the overlap relationship include commonpixels on the screen, allowing error correction to be achieved by usingthe strong pixel correlation. Referring to the overlapping pixelcorrelation, in the example of FIG. 4, common pixels in the overlaprelationship have the following relationship: $\begin{matrix}{\begin{pmatrix}\begin{matrix}\begin{matrix}a_{03} \\a_{13}\end{matrix} \\a_{23}\end{matrix} \\a_{33}\end{pmatrix} = \begin{pmatrix}\begin{matrix}\begin{matrix}b_{00} \\b_{10}\end{matrix} \\b_{20}\end{matrix} \\b_{30}\end{pmatrix}} & (1)\end{matrix}$

If the transform base of DCT is represented by a matrix D and if pixeldata on both sides of Equation (1) are represented by coefficient data,then the following Equations (2) and (3) are obtained: $\begin{matrix}{{\begin{pmatrix}a_{03} \\a_{13} \\a_{23} \\a_{33}\end{pmatrix} = {{D^{T}\begin{pmatrix}A_{00} & A_{01} & A_{02} & A_{03} \\A_{10} & A_{11} & A_{12} & A_{13} \\A_{20} & A_{21} & A_{22} & A_{23} \\A_{30} & A_{31} & A_{32} & A_{33}\end{pmatrix}}d_{3}}},{and}} & (2) \\{\begin{pmatrix}b_{00} \\b_{10} \\b_{20} \\b_{30}\end{pmatrix} = {{D^{T}\begin{pmatrix}B_{00} & B_{01} & B_{02} & B_{03} \\B_{10} & B_{11} & B_{12} & B_{13} \\B_{20} & B_{21} & B_{22} & B_{23} \\B_{30} & B_{31} & B_{32} & B_{33}\end{pmatrix}}{d_{0}.}}} & (3)\end{matrix}$

Substituting Equations (2) and (3) into Equation (1) yields thefollowing Equation (4): $\begin{matrix}{{\begin{pmatrix}A_{00} & A_{01} & A_{02} & A_{03} \\A_{10} & A_{11} & A_{12} & A_{13} \\A_{20} & A_{21} & A_{22} & A_{23} \\A_{30} & A_{31} & A_{32} & A_{33}\end{pmatrix}d_{3}} = {\begin{pmatrix}B_{00} & B_{01} & B_{02} & B_{03} \\B_{10} & B_{11} & B_{12} & B_{13} \\B_{20} & B_{21} & B_{22} & B_{23} \\B_{30} & B_{31} & B_{32} & B_{33}\end{pmatrix}{d_{0}.}}} & (4)\end{matrix}$

The matrix D is the transform base of DCT, and it is represented in theform of following Equation (5): $\begin{matrix}{D = {\begin{pmatrix}d_{00} & d_{01} & d_{02} & d_{03} \\d_{10} & d_{11} & d_{12} & d_{13} \\d_{20} & d_{21} & d_{22} & d_{23} \\d_{30} & d_{31} & d_{32} & d_{33}\end{pmatrix} = {\begin{pmatrix}d_{0} & d_{1} & d_{2} & d_{3}\end{pmatrix}.}}} & (5)\end{matrix}$

Equation (4) obtained above represents that eight coefficient datahaving the same vertical frequency in two coding blocks with the overlaprelationship, for example, (A_(m0), A_(m1), A_(m2), A_(m3)) and (B_(m0),B_(m1), B_(m2), B_(m3)), are linearly dependent on each other (wherem=0, 1, 2 or 3). This fact shows that even if one of the eightcoefficients is erroneous (hereinafter, an erroneous coefficient will bereferred to as error coefficient) and correct coefficient data havingbeen dropped out, the missing error coefficient can be restored by usingthe remaining seven coefficient data. For example, when an error of A₁₀is detected, A₁₀ can be restored by using the data of the other codingblock, (B₁₀, B₁₁, B₁₂, B₁₃) with use of Equation (4).

However, the number of error coefficients depends on circumstancesduring recording and reproducing processes, and it would not necessarilybe the above case that only one error coefficient is present among theeight coefficient data. Therefore, the state of occurrence of errorcoefficients is analyzed by the error-correction control circuit 14according to decision conditions as shown below for each of the eightcoefficient data having the same vertical frequency, and the result ofthis analysis is outputted to the error correcting circuit 15 as anerror-correction control signal. The predetermined specified number oforder to be used for the decision is set to 2. However, another numberof order, e.g, 1, may be used.

The error correction signal can be classified into the following threetypes, and they are decided as follows:

(a) Zero error correction signal: With respect to two coding blocks inthe overlap relationship, a zero error correction signal is outputtedfor the error coefficients among eight coefficient data having the samevertical frequency component if the error coefficients are present amongthe eight coefficient data and if the numbers of order of horizontalfrequency of the error coefficients are all 2 or more.

(b) Overlap correlation error correction signal: With respect to twocoding blocks in the overlap relationship, if error coefficients arepresent among the eight coefficient data having the same verticalfrequency component and if only one error coefficient whose number oforder of horizontal frequency is less than 2 exists among the errorcoefficients, then the other error coefficients are set to zero forerror correction. Then, the error coefficient whose number of order isless than 2 is corrected by using the overlap pixel correlation. Thisinstruction is outputted as an overlap correlation error correctionsignal.

(c) Three-dimensional error correction signal: In a case other than theabove ones (a) and (b), that is, when a plurality of error coefficientswhose number of order of horizontal frequency is less than 2 are presentamong the eight coefficient data having the same vertical frequencycomponent, it is decided which of the two coding blocks in the overlaprelationship the plurality of error coefficients are present in. Then,for a coding block including an error coefficient whose number of orderis less than 2, a three-dimensional error correction signal is outputtedinstructing for the replacement of all the coefficient data in thecoding block with coefficient data in a coding block preceding by onefield and located in the same region on the screen which includes allthe coefficient data. Further, a zero error correction signal isoutputted for a coding block in which error coefficients whose number oforder is less than 2 are present only in one of the coding blocks in theoverlap relationship and in which coefficients whose number of order offrequency is less than 2 are included.

The error correcting circuit 15 performs error correction as explainedbelow according to an input error-correction control signal:

(d) When an overlap correlation error correction signal is inputted asan error-correction control signal, error correction is performed byusing the aforementioned pixel correlation of overlap boundary pixels,for an error coefficient whose number of order of horizontal frequencyis the lowest among the error coefficients having the same verticalfrequency in the two coding blocks in the overlap relationship, and theremaining error coefficients are changed to zeroes.

(e) When a zero error correction signal is inputted as anerror-correction control signal, all the error coefficients included inthe coefficient data having the same vertical frequency in the twocoding blocks in the overlap relationship are changed to zeroes. This isbecause these coefficient data have only small influence on the imagequality.

(f) When a three-dimensional error correction signal is inputted as anerror-correction control signal, all the coefficient data in the codingblock are replaced with coefficient data in a coding block preceding byone field in the same region on the screen. In a different way,coefficient data of the previous frames may be used for the replacement.

As described above, according to this embodiment, in the division ofdigital video signals into coding blocks, they are divided into codingblocks so that coding blocks adjacent to each other on the screen areoverlapped so as to share one pixel of their boundary. Then, coefficientdata in two coding blocks in the overlap relationship are recorded toseparate regions on the recording medium 8 for each coefficient group G.Then, for example, even if a region where a coefficient group G_(A0) ofa coding block A has been recorded is not correctly reproduced due to adropout or other reasons, error correction is possible by using theother coefficient groups and by using Equation (4) for the errorcoefficient.

Further, as shown in FIG. 5, in the process of recording, a plurality ofrecording groups in two coding blocks in the overlap relationship arenot recorded simultaneously to the same recording track. Therefore, evenif narrowing of a track or other phenomenon has occurred at an IN or OUTpoint during the edition of digital video signals, it would be only onecoefficient group G that drops out due to an error. Thus, errors can becorrected with high accuracy.

Furthermore, in this recording and reproducing apparatus, it is arrangedthat the coefficient group G including coefficient data corresponding tothe horizontal low frequency region in two coding blocks in the overlaprelationship are not recorded simultaneously by the same recording head7. Further, it is arranged that the coefficient group G includingcoefficient data corresponding to the same horizontal frequency in thetwo coding blocks is not recorded simultaneously by the same recordinghead 7. These aspects have already been described with reference to FIG.5. Therefore, a coefficient group that may drop out as an error due tosuch a reason as damage of one recording head or reproducing head is acombination of low frequency and high frequency components in two codingblocks in the overlap relationship. In the example shown in FIG. 5,there is a possibility that data of the groups G_(A0), G_(B2), G_(B1)may result in an error upon a failure of the recording head 7 a, wherethese groups have horizontal frequencies different from one another.Accordingly, coefficient groups (e.g., groups G_(A0) and G_(B0))including coefficient data corresponding to the same horizontalfrequency in the two coding blocks will never result in an error at thesame time. Thus, the error correction using pixel correlation ofEquation (4) is enabled.

As a result, the possibility that a plurality of low frequencycomponents showing important characteristic properties of the image inthe coding blocks may drop out simultaneously is minimized, so that,substantially, almost no three-dimensional error correction signals areoutputted in the error-correction control circuit 14. When inter-fielderror correction is performed, coding blocks different in time from oneanother would be mixed on the same field so that awkward reproductionmight occur particularly to regions on the digital video signals relatedto a motion. However, such inter-field error correction can be avoidedin this embodiment as much as possible.

Low frequency components in the digital video signals showcharacteristic properties of the digital video signals and have most ofthe information included therein. On the other hand, high frequencycomponents thereof have statistical properties such that they show finerinformation therein and that their amplitudes are small. Therefore, theerror-correction control circuit 14 can analyze, in terms of the visualsense of an image viewer, the information lost in the reproductionprocess due to an error in coding block by analyzing the numbers oferror coefficients included in the low frequency components whose numberof order of horizontal frequency is less than 2 and in the highfrequency components whose number of order of horizontal frequency is 2or more. As a result, image quality can be subjected to sufficient errorcorrection even if a plurality of error coefficients have occurred amongthe eight coefficient data having the same vertical frequency in twocoding blocks in the overlap relationship.

For example, when the region of a coefficient group G_(A2) in the codingblock A has resulted in an error during the recording or reproducingprocess, the error-correction control circuit 14 outputs a zero errorcorrection signal based on this state where error coefficients aregenerated. The error coefficients are more likely to have smallamplitudes by nature, so that sufficient image quality subjected toerror correction can be acquired by error correction to set all theerror coefficients to zeroes.

When the recording head 7 or the reproducing head 9 is damaged so thatcoefficient data contained in the coefficient group G_(x0) and thecoefficient group G_(x2) have resulted in an error, the error-correctioncontrol circuit 14 outputs an overlap correlation error correctionsignal. However, even after error correction has been performed so thaterror coefficients contained in the coefficient group G_(x2) of highfrequency components, which have small amplitudes, are changed tozeroes, Equation (4) can be developed without causing any large error tooccur to the error coefficients contained in the coefficient groupG_(x0) of low frequency components, which have large amplitudes bynature. Thus, the error coefficients contained in the coefficient groupG_(x0) can be corrected so that image quality corrected sufficiently oferrors can be acquired.

Also, although this embodiment has been described on the assumption thatthe number of order showing horizontal high frequency components isconstant regardless of vertical frequency, it is more advantageous thatthe number of order is varied with the number of order of verticalfrequency. For example, the number of order showing horizontal highfrequency components is set to become smaller as the vertical frequencybecomes higher, taking into consideration a property that coefficientdata have smaller amplitudes at higher frequency components. In anexample, when the number of order of vertical frequency is 0, the numberof order showing horizontal high frequency components is set to 2; whenthe number of order of vertical frequency is 1 or more, the number oforder showing horizontal high frequency components is set to 1. As aresult, in regions of higher vertical frequencies, which are of lessvisual importance, three-dimensional error correction signals becomeunlikely to occur so that the image quality can be further improved witherror correction in regions related to motion in the digital videosignals.

In addition, in this embodiment, coefficient data within coding blocksoverlapping in the horizontal direction on the screen are divided intogroups in the unit of coefficient data that do not have any identicalvertical frequency component, and error correction is performed for eachgroup. Otherwise, it is needless to say that equivalent error correctioncan be achieved even when coefficient data within coding blocksoverlapping in the vertical direction on the screen are divided intogroups in the unit of coefficient data that do not have any identicalhorizontal frequency component and error correction is done in the unitof the group. Further, though this embodiment adopts the size of acoding block, the intra-field compression coding and the recording onsix tracks per field by three recording heads, it is only an example,and the present invention is not limited thereto. For example, though a4×4 block has been adopted in this embodiment as the size of a codingblock, similar error correction can also be achieved for any size of8×8, 8×4 or the like. Further, the frequency transform is not limited tothe discrete cosine transform, and similar error correction can beachieved also with Hadamard transform, Slant transform, Legendretransform and the like.

As described above, according to the present invention, when an erroroccurs, a possibility that all the coefficient data within a block maybe dropped out and be lost in the block during reproduction can beprevented. Further, the stability of reproduced images can be enhancedby performing error correction with the use of coefficient data ofoverlapping neighboring blocks, and this is a great practical advantage.

The present invention has been described in detail by way of anembodiment thereof hereinabove. However, the present invention is notlimited to the above-described embodiment, and various changes andmodifications may be made without departing from the spirit of theinvention as set forth in the appended claims.

What is claimed is:
 1. A recording and reproducing apparatus with imagecoding, said recording and reproducing apparatus comprising: overlapblocking means for dividing input digital video signals into data intwo-dimensional coding blocks, each of the coding blocks having firstand second axes and comprising of a plurality of pixel data such thatneighboring coding blocks are overlapped with each other so as to shareat least one boundary pixel in an axis direction; orthogonal transformmeans for performing two-dimensional frequency conversion for theplurality of pixel data of the coding blocks obtained by said overlapblocking means to generate coefficient data of individual frequencies;shuffling means for dividing the coefficient data obtained by saidorthogonal transform means into a plurality of coefficient groups sothat the coefficient groups do not have a same frequency component ofthe second axis at least in a frequency region in the coding blockscorresponding to low frequency components of the first axis; recordingmeans for locating recording data so that the plurality of coefficientgroups in a same coding block obtained by said shuffling means arerecorded so as to be dispersed into separate regions on a recordingmedium, and for adding error correction codes for the coefficient data;reproducing and decoding means for reproducing the recording data fromthe recording medium, for decoding the coefficient data of the pluralityof coefficient groups, individually, and for detecting a presence orabsence of an error of coefficient data; deshuffling means forintegrating coefficient data outputted from said reproducing anddecoding means into the coding blocks; error-correction control meansfor outputting an error-correction control signal based on a number oferrors in a coding block and a number of order of two-dimensionalfrequencies of coefficient data to which errors have occurred when anerror has been detected by said reproducing and decoding means; errorcorrecting means for correcting errors of the coefficient data by anerror correcting method selected by the error-correction control signalof said error-correction control means with respect to the coding blockin which errors have been detected; inverse orthogonal transform meansfor decoding the coefficient data that have been corrected of errors bysaid error correcting means to pixel data by frequency inverseconversion; relocating means for generating digital video signals byrelocating the pixel data to the coding blocks; a rotary cylinderoperable to helically scan the recording medium; and signal recordingmeans having a plurality of recording heads attached to said rotarycylinder, wherein said recording means outputs the recording data tosaid signal recording means so that coefficient groups includingcoefficient data corresponding to low frequency components of the firstaxis in at least two overlapped coding blocks are not recorded by a samerecording head.
 2. The recording and reproducing apparatus according toclaim 1, wherein said recording means locates the recording data so thatthe plurality of coefficient groups belonging to the same coding blockare not recorded to a same recording track on the recording medium.
 3. Arecording and reproducing apparatus with image coding, said recordingand reproducing apparatus comprising: overlap blocking means fordividing input digital video signals into data in two-dimensional codingblocks, each of the coding blocks having first and second axes andcomprising of a plurality of pixel data such that neighboring codingblocks are overlapped with each other so as to share at least oneboundary pixel in an axis direction; orthogonal transform means forperforming two-dimensional frequency conversion for the plurality ofpixel data of the coding blocks obtained by said overlap blocking meansto generate coefficient data of individual frequencies; shuffling meansfor dividing the coefficient data obtained by said orthogonal transformmeans into a plurality of coefficient groups so that the coefficientgroups do not have a same frequency component of the second axis atleast in a frequency region in the coding blocks corresponding to lowfrequency components of the first axis; recording means for locatingrecording data so that the plurality of coefficient groups in a samecoding block obtained by said shuffling means are recorded so as to bedispersed into separate regions on a recording medium, and for addingerror correction codes for the coefficient data; reproducing anddecoding means for reproducing the recording data from the recordingmedium, for decoding the coefficient data of the plurality ofcoefficient groups, individually, and for detecting a presence orabsence of an error of coefficient data; deshuffling means forintegrating coefficient data outputted from said reproducing anddecoding means into the coding blocks; error-correction control meansfor outputting an error-correction control signal based on a number oferrors in a coding block and a number of order of two-dimensionalfrequencies of coefficient data to which errors have occurred when anerror has been detected by said reproducing and decoding means; errorcorrecting means for correcting errors of the coefficient data by anerror correcting method selected by the error-correction control signalof said error-correction control means with respect to the coding blockin which errors have been detected; inverse orthogonal transform meansfor decoding the coefficient data that have been corrected of errors bysaid error correcting means to pixel data by frequency inverseconversion; relocating means for generating digital video signals byrelocating the pixel data to the coding blocks; a rotary cylinderoperable to helically scan the recording medium; and signal recordingmeans having a plurality of recording heads attached to said rotarycylinder, wherein said recording means outputs the recording data sothat coefficient groups including coefficient data corresponding to asame frequency component of the first axis in two coding blocks in anoverlapped relationship are not recorded by a same recording head. 4.The recording and reproducing apparatus according to claim 3, whereinsaid recording means locates the recording data so that the plurality ofcoefficient groups belonging to the same coding block are not recordedto a same recording track on the recording medium.
 5. A recording andreproducing apparatus with image coding, said recording and reproducingapparatus comprising: overlap blocking means for dividing input digitalvideo signals into data in two-dimensional coding blocks, each of thecoding blocks having first and second axes and comprising of a pluralityof pixel data such that neighboring coding blocks are overlapped witheach other so as to share at least one boundary pixel in an axisdirection; orthogonal transform means for performing two-dimensionalfrequency conversion for the plurality of pixel data of the codingblocks obtained by said overlap blocking means to generate coefficientdata of individual frequencies; shuffling means for dividing thecoefficient data obtained by said orthogonal transform means into aplurality of coefficient groups so that the coefficient groups do nothave a same frequency component of the second axis at least in afrequency region in the coding blocks corresponding to low frequencycomponents of the first axis; recording means for locating recordingdata so that the plurality of coefficient groups in a same coding blockobtained by said shuffling means are recorded so as to be dispersed intoseparate regions on a recording medium, and for adding error correctioncodes for the coefficient data; reproducing and decoding means forreproducing the recording data from the recording medium, for decodingthe coefficient data of the plurality of coefficient groups,individually, and for detecting a presence or absence of an error ofcoefficient data; deshuffling means for integrating coefficient dataoutputted from said reproducing and decoding means into the codingblocks; error-correction control means for outputting anerror-correction control signal based on a number of errors in a codingblock and a number of order of two-dimensional frequencies ofcoefficient data to which errors have occurred when an error has beendetected by said reproducing and decoding means; error correcting meansfor correcting errors of the coefficient data by an error correctingmethod selected by the error-correction control signal of saiderror-correction control means with respect to the coding block in whicherrors have been detected; inverse orthogonal transform means fordecoding the coefficient data that have been corrected of errors by saiderror correcting means to pixel data by frequency inverse conversion;and relocating means for generating digital video signals by relocatingthe pixel data to the coding blocks, wherein said error-correctioncontrol means outputs a zero error correction signal for replacing anerror coefficient with a zero when a plurality of error coefficients arepresent in coefficient data having a same frequency in the second axisin two coding blocks in an overlapped relationship and when all theerrors are present at numbers of order higher than a specified number oforder, said error-correction control means outputs an overlapcorrelation error correction signal for instructing error correction bymaking use of a correlation between coefficient data of neighboringcoding blocks when a plurality of errors are present in coefficient datahaving a same frequency in the second axis in two coding blocks in theoverlapped relationship and when only one error coefficient is presentat a number of order lower than a specified number of order, saiderror-correction control means outputs a three-dimensional errorcorrection signal for replacing all the coefficient data in the codingblock with coefficient data of a preceding frame or preceding field whentwo or more errors are present at numbers of order lower than aspecified number of order for each coding block and when errorcoefficients are present on a side of lower numbers of order than aspecified number of order in coefficient data having a same frequency inthe second axis for each coding block, and said error-correction controlmeans outputs a zero error correction signal for replacing the errorcoefficients with zeroes for coding blocks when no error coefficientsare present in the coding blocks on a side of lower numbers of orderthan a specified number of order on a coding block basis.
 6. Therecording and reproducing apparatus according to claim 5, wherein saidrecording means locates the recording data so that the plurality ofcoefficient groups belonging to the same coding block are not recordedto a same recording track on the recording medium.
 7. The recording andreproducing apparatus according to claim 5, wherein when the overlapcorrelation error correction signal is received from saiderror-correction control means, said error correcting means corrects, byusing a correlation of pixels overlapped in the first axis on a screen,error coefficients corresponding to frequency components of a lowestregion of the first axis out of a plurality of error coefficients incoefficient data having a same frequency in the second axis in twocoding blocks in the overlapped relationship and corrects remainingerror coefficients as zeroes, and corrects to zeroes all of theplurality of error coefficients among coefficient data having a samefrequency in the second axis in two coding blocks in the overlappedrelationship when the zero error correction signal is received from saiderror-correction control means, and when the three-dimensional errorcorrection signal is received, said error correcting means replaces allthe coefficient data within a coding block for correction withcoefficient data of a corresponding coding block preceding by one fieldor one frame and constituting a same region on the screen.
 8. Therecording and reproducing apparatus according to claim 5, wherein saiderror-correction control means changes a specified number of order offrequencies in the first axis in response to a number of order offrequencies in the second axis.